The present invention relates in general to a power supply and more particularly to an apparatus and method for controlling a power supply by applying a temporary load to the power supply in order to generate a control signal.
Dual switched-mode power supplies are commonly used for electronic devices such as televisions that generally operate at two power levels, a high power level during normal operation and a low power level during standby operation. For example, for a television the high power level corresponds to providing power to the picture tube, receiver, speaker, and control circuitry, and the low power level corresponds to supplying power only to the control circuitry so that the television remains in a standby mode. A large, main switched-mode power supply is used to provide power to the picture tube, receiver, and speaker, and a small, auxiliary switched-mode power supply is used to supply power to the control circuitry.
The control circuitry for the television generally comprises a microprocessor to control the various functions of the television, memory to store operating information like the last volume level and last channel watched by the user, and a detector to receive signals from a remote control unit. The use of the dual power supply conserves energy when the television is in the standby mode.
To operate the television, the user generally presses a button on the remote control, which transmits a signal to the detector in the television. The detector transmits the signal to the microprocessor, which decodes the signal and performs the requested function, for example, turning on or off the television.
When the user has requested that the television turn off, the microprocessor turns off the main power supply, leaving the auxiliary power supply on. In prior televisions, because the control circuitry is located on the isolated side of the auxiliary power supply, complex circuitry such as a mechanical relay or a combination of signals on a secondary-side optocoupler, which provides a normal feedback plus a forced standby signal in response to the user requesting the shutdown of the television, is used to communicate the shutdown command from the control circuitry to the main power supply. It would be desirable to communicate this command using simplified circuitry. Accordingly, there is a need for an improved way in a dual power supply configuration to communicate a shutdown command to the main power supply from control circuitry powered by the auxiliary power supply.